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<html lang="en">

<head>
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            margin: 10px;
            width: 100px;
            height: 100px;
            background-color: #000;
        }
    </style>
</head>

<body>
    <div class="container">
        <div class="box"></div>
        <div style="display:flex">
            <!-- div并不具备创建BFC的能力，怎么让他具备呢？
                BFC 拥有独立的不受外界影响的格式化上下文:
                FFC = BFC + flex
                overflow:hidden
            -->
            <div class="box"></div>
        </div>
    </div>
</body>

</html>